Translation of fan fiction in Spanish:

noun

As its name suggests, fan fiction is fiction written by those who are great enthusiasts for a particular writer's work, and have borrowed his or her characters and contexts to write fiction of their own.

Unless you're the kind of person who spends their weekends writing fan fiction about video game characters this won't matter too much.

As a fledgling writer, I really wouldn't spend too much time worrying that people will write fan fiction with your characters in.